1. Field of the Invention
A detection device used to detect the magnetic field strength of an object that can affect an implanted medical device.
2. Discussion of Related Art
Modern medical device technology provides a number of devices that can be implanted in a patient. The implanted devices monitor a patient's condition, regulate bodily functions, and/or administrate medication to the patient. Examples of implanted devices include pacemakers and medication pumps. Many times there is a need to alter the settings on the device, for example a pump, after it has been implanted. One method to accomplish this, without removing the device, is to use magnetically activated switches. These switches respond to magnetic fields to, for example, open or close valves on a pump. In order to affect such a change, medical personnel place a magnet of specific strength over the implanted device. The magnetic field strength needed to change the setting is orders of magnitude greater than the Earth's magnetic field of 0.3 to 0.6 gauss.
Magnetic switch technology is a simple and effective way to alter the settings on the implanted devices, but it has some drawbacks. Numerous commercial products contain magnets of sufficient strength to alter the settings or otherwise adversely impact the operation of an implanted device. Thus, a patient can unknowingly intersect the magnetic field of one of these objects and alter the operation of the implanted device. Common items like cell phones, speakers and children's toys all may contain magnets of sufficient strength at a close enough distance to alter the operation of the implanted device. Currently, there is no device available that allows a patient to easily and quickly determine if such an item has a magnetic field strong enough to alter the operation of an implanted device.
Numerous prior art devices provide the ability to detect weak magnetic fields. U.S. Pat. No. 2,123,045 to Hoare detects weak magnetic fields to determine the location of metal pipes burred in the ground or walls. Hoare uses one or more ordinary compasses and/or a bar magnet near the compass needle to compensate for the Earth's magnetic field. A user of Hoare's device is required to cause the device to traverse the location of a suspected weak magnetic mass and watch for deflections of the needle. Slight deflections of the needle indicate that the user is nearing the weak magnetic mass and the point of greatest deflection indicates the probable location of the mass. However, Hoare's device is not designed to alert a user to the strength of the magnetic field; but, instead is designed to indicate its location. Hoare is compensating against the Earth's magnetic field because he is looking for very weak fields, not fields of a particular strength.
U.S. Pat. No. 3,349,323 to Mullen discloses using a reed switch biased by two magnetic fields, one from a permanent magnet and another from an electromagnet. The electromagnet is operated by an electrical current that is used to generate a magnetic field in a coil positioned near or around the reed switch. The electrical field generated by the coil is designed to counter the magnetic field generated by the permanent magnetic and acts to keep the switch open, while the permanent magnetic field biases it closed. As Mullen's device approaches a magnetic source, the switch oscillates open and closed with different frequencies, depending on whether the source overcomes the permanent magnetic field or the electromagnetic field. The device senses the oscillations and the frequency of the oscillations is an indication of strength and location. However, this device is complex and requires an electromagnetic circuit, including a battery, a switch, and a milliampmeter to detect the small oscillations of the switch. Calibration of the device is difficult, matching the magnetic field of the permanent magnet, the magnetic resistance of the switch, the power across the coil to form the electromagnetic field and the distance between the three. Further, since the device is powered, it is unreliable due to power spikes or dips that can alter the calibration of the device.
Thus, there is a need in the art for a device that can detect the strength of a strong magnetic field and that is easily calibrated without the use of circuits and battery power to add to the reliability of the device.